This invention relates to an apparatus for detecting whether or not a paper sheet is flat.
Such detection is needed for automatically handling postal matter. Recently, in Japan, the various handling steps of impressing a cancellation mark and distinguishing the destination have been automated. Regular Japanese domestic mail prior to sorting in a post office may be in a mixed condition where the mail is facing the wrong way or where the mail is upside down. Therefore, before a cancellation mark is impressed on a stamp, it is necessary to straighten out the mail according to whether or not it is face up and whether or not the mail is upside down. Usually, a stamp or mailing label is usually applied in a prescribed location of the envelope in regards to Japanese domestic mail only. (In Japan, the upper right-hand corner of the front side is used.) Therefore, the sorting of the Japanese domestic mail can be carried out in accordance with the detected position of a stamp or mailing label. The Japanese domestic postage stamp or mailing label is generally treated with a chemical process (for instance, to ensure a chemical reaction with ultraviolet rays), thereby facilitating the detection of a postage stamp or mailing label while the mail is being transferred one by one through a sorting machine. While the mail is being carried through the sorting machine, it is impressed with a cancellation mark and is straightened out by means of a front and back reversing machine or an up and down reversing machine. The postal district numbers written in the prescribed corner of the postal matter are read to sort the mail in accordance with its destination. Where a postal district number is not written in the prescribed position of the surface of the postal matter a postal district number is impressed by a post office clerk using a bar code printer in the form of a bar code in a prescribed position on the surface of the postal matter (for example, the lower righthand corner of the surface thereof) after straightening out the postal matter. The bar codes are read as postal district numbers using a bar code reader, thereby ensuring a prominent saving of labor and a quick delivery of the postal matter.
Outside of Japen, however, postage stamps or mailing labels are not chemically treated. Therefore, the automatic sorting of postal matter relying on the detection of a chemically processed postage stamp or mailing mark has failed to be affected. Therefore, it is desired to detect whether or not the mail is face up, and whether or not the top is up without using the postage stamp or mailing label. Aside from the postage stamp or mailing label, the flap of an envelope is also at a prescribed position within the mailing matter. However, there is no method of exactly detecting the position of the flap. Therefore, postal matter must be manually straightened out by a post office clerk. Thereafter, the postal matter is set in a bar code printer or postal district number reader. The manual work of straightening out the mail consumes a considerable amount of time, obstructing the quick delivery of the postal matter.